SELECTIVITY OF AMINO-ACID TRANSMITTERS ACTING AT N-METHYL-D-ASPARTATEAND AMINO-3-HYDROXY-5-METHYL-4-ISOXAZOLEPROPIONATE RECEPTORS

The endogenous neurotransmitter candidates L-aspartate, L-cysteine sul finate (CSA), L-glutamate, L-homocysteate (HCA), and the endogenously occurring analogue quinolinate were compared in terms of potency, maxi mal activity, and selectivity for steady state activation of N-methyl- D-aspartate (NMDA) and non-NMDA RS)-amino-3-hydroxy-5-methyl-4-isoxazo lepropionate (AMPA)] types of glutamate receptors expressed in Xenopus oocytes injected with mRNA isolated from rat brain (minus cerebellum) . Selective activation of NMDA receptors was achieved by deleting Mg2 and including 3-10-mu-M glycine in the perfusion medium and by applyi ng ligands in the presence of 30-mu-M quisqualate, which blocks the AM PA receptor and desensitizes the oocyte's own Ca2+-dependent Cl- curre nt. Oocytes were voltage clamped, and steady state inward currents wer e measured in response to perfusion with agonists at known concentrati ons. Under the NMDA receptor-preferring condition, the potency rank or der was L-glutamate (EC50 = 2.2-mu-M, 95% confidence interval = 1.4-3. 6-mu-M) > L-aspartate (13-mu-M) = HCA (13-mu-M) > CSA (59-mu-M) > quin olinate (greater-than-or-equal-to 7200-mu-M). All amino acids tested e voked similar maximal currents, which were 120-159% that of NMDA itsel f. The Hill coefficient was greater than 1 for all agonists except L-H CA (0.6), which might reflect heterogeneity of NMDA receptors expresse d. This was supported by the finding that glycine was more potent in c ombination with HCA than NMDA, in activating NMDA receptors. To study the activity of agonists at AMPA receptors, glycine and quisqualate we re omitted and 1 mM Mg2+ was included to block NMDA receptors. Ca2+-de pendent Cl- currents activated by L-glutamate were prevented by inclus ion of 0.4 M ethylene glycol bis(beta-aminoethyl ether)-N,N,N',N'-tetr aacetic acid in the recording electrode. All amino acids were less pot ent at AMPA receptors than at NMDA receptors; the potency rank order f or steady state activation of AMPA receptors was L-glutamate (EC50 = 1 1-mu-M, 95% confidence interval = 7.3-18-mu-M) > HCA (430-mu-M) > CSA (3300-mu-M). L-Aspartate and quinolinate produced little or no inward current even up to 10 mM, i.e., were inactive at forebrain AMPA recept ors. The maximal currents activated by all amino acids at steady state were 5-10% that of kainate, presumably due to severe desensitization of the AMPA receptor by the natural agonists. These results are consis tent with L-glutamate acting as a mixed agonist at both AMPA and NMDA synaptic receptors and L-aspartate being involved exclusively in NMDA receptor-mediated synapses.